The present invention relates to a new and distinct variety of nectarine tree named ‘NJF20’. The new tree resulted from crossing a proprietary nectarine seedling selection ‘L5-225-01250’ (non-patented) as the seed parent with a nectarine seedling selection ‘Y153-69’ (non-patented) as the pollen parent. The new variety differs from seed parent ‘L5-225-01250’ in that the new variety produces flat fruit with yellow flesh that are sweeter and ripen seven weeks earlier. In contrast, the seed parent ‘L5-225-01250’ produces round fruit with white flesh that tend to have a milder flavor. The new variety differs from pollen parent ‘Y153-69’ in that the new variety produces larger fruit that may be slightly more tolerant to bacterial leaf spot. The resulting tree was selected when growing in a cultivated area as the 23rd tree in the 13th row of Block H at a fruit research farm in Cream Ridge, N.J.